M. Richetta

STARDUST experimental campaign and numerical simulations: influence of obstacles and temperature on dust resuspension in a vacuum vessel under LOVA

Activated dust mobilization during a Loss of Vacuum Accident (LOVA) is one of the safety concerns for the International Thermonuclear Experimental Reactor (ITER). Intense thermal loads in fusion devices occur during plasma disruptions, edge localized modes and vertical displacement events. They will result in macroscopic erosion of the plasma facing materials and consequent accumulation of activated dust into the ITER vacuum vessel (VV). These kinds of events can cause dust leakage outside the VV that represents a high radiological risk for the workers and the population. A small facility, Small Tank for Aerosol Removal and Dust (STARDUST), was set up at the ENEA Frascati laboratories to perform experiments concerning the dust mobilization in a volume with the initial conditions similar to those existing in ITER VV. The aim of this work was to reproduce a low pressurization rate (300?Pa?s?1) LOVA event in a VV due to a small air leakage for two different positions of the leak, at the equatorial port level and at the divertor port level, in order to evaluate the influence of obstacles and walls temperature on dust resuspension during both maintenance (MC) and accident conditions (AC) (Twalls = 25??C MC, 110??C AC). The dusts used were tungsten (W), stainless steel 316 (SS316) and carbon (C), similar to those produced inside the vacuum chamber in a fusion reactor when the plasma facing materials vaporize due to the high energy deposition. The experimental campaign has been carried out by introducing inside STARDUST facility an obstacle to simulate the presence of objects, such as divertor. In the obstacle a slit was cut to simulate the limiter?divertor gap inside ITER VV. In this paper experimental campaign results are shown in order to investigate how the divertor and limiter?divertor gap influence dust mobilization into a VV. A two-dimensional (2D) modelling of STARDUST was made using the CFD commercial code FLUENT, in order to get a preliminary overview of the fluid dynamics behaviour during a LOVA event and to justify the mobilization data. In addition, a numerical model was developed to compare numerical results with experimental ones.

Safety Analysis in Large Volume Vacuum Systems Like Tokamak: Experiments and Numerical Simulation to Analyze Vacuum Ruptures Consequences

The large volume vacuum systems are used in many industrial operations and research laboratories. Accidents in these systems should have a relevant economical and safety impact. A loss of vacuum accident (LOVA) due to a failure of the main vacuum vessel can result in a fast pressurization of the vessel and consequent mobilization dispersion of hazardous internal material through the braches. It is clear that the influence of flow fields, consequence of accidents like LOVA, on dust resuspension is a key safety issue. In order to develop this analysis an experimental facility is been developed: STARDUST. This last facility has been used to improve the knowledge about LOVA to replicate a condition more similar to appropriate operative condition like to kamaks. By the experimental data the boundary conditions have been extrapolated to give the proper input for the 2D thermofluid-dynamics numerical simulations, developed by the commercial CFD numerical code. The benchmark of numerical simulation results with the experimental ones has been used to validate and tune the 2D thermofluid-dynamics numerical model that has been developed by the authors to replicate the LOVA conditions inside STARDUST. In present work, the facility, materials, numerical model, and relevant results will be presented.

UMEL: A new regression tool to identify measurement peaks in LIDAR/DIAL systems for environmental physics applications

Recently, surveying large areas in an automatic way, for early detection of both harmful chemical agents and forest fires, has become a strategic objective of defence and public health organisations. The Lidar and Dial techniques are widely recognized as a cost-effective alternative to monitor large portions of the atmosphere. To maximize the effectiveness of the measurements and to guarantee reliable monitoring of large areas, new data analysis techniques are required. In this paper, an original tool, the Universal Multi Event Locator, is applied to the problem of automatically identifying the time location of peaks in Lidar and Dial measurements for environmental physics applications. This analysis technique improves various aspects of the measurements, ranging from the resilience to drift in the laser sources to the increase of the system sensitivity. The method is also fully general, purely software, and can therefore be applied to a large variety of problems without any additional cost. The potential of the proposed technique is exemplified with the help of data of various instruments acquired during several experimental campaigns in the field.

Raman water vapour concentration measurements for reduction of false alarms in forest fire detection

Forest fires can be the cause of environmental catastrophe, with the natural outcomes of serious ecological and economic damages, together with the possibility to endanger human safety. At the aim to reduce this catastrophe several author have been shown that the Laser light scattering can be uses to reveals the particulate emitted in the smoke. Infact experimental and theoretical investigations have shown that lidar is a powerful tool to detect the tenuous smoke plumes produced by forest fires at an early stage. In early 90s Arbolino and Andreucci have shown the theoretical possibility to detect the particulate emitted in atmosphere from smoke forest fire. Vilar at all have shown experimentally the possibility to measure the density variation in atmosphere due to plume emitted in forest fire event. Gaudio at all. have already shown that it is possible to evaluate water vapor emitted in smoke of vegetable fuel using a CO2 dial system. In this paper a theoretical model to evaluate the capabilities of a lidar system in fire surveillance of wooded areas will be presented. In particular we intend propose a technique to minimizing the false alarm in the detection of forest fire by lidar based on a measurement of second components emitted in a combustion process. Usually to detect a fire alarm a rapid increase of aerosol amount is measured. If the backscattering signal report a peak, the presences of a forest fire will be probable. Our idea to confirm this hypothesis is measure the second components emitted in a forest fire at the aim to minimize the false alarm. The simulated measurements of the humidity amount within the smoke plume will be carried out by means of Raman analysis. Fixing the burning rate of the vegetable-fuels, the maximum range of detection will be evaluated.

Early detection of small forest fire by dial technique

Forest fires can be the cause of serious environmental and economic damages. For this reason considerable effort has been directed toward forest protection and fire fighting. The means traditionally used for early fire detection mainly consist in human observers dispersed over forest regions. A significant improvement in early warning capabilities could be obtained by using automatic detection apparatus. In order to early detect small forest fires, the use of a dial system will be considered. A first evaluation of the lowest detectable concentration will be estimated by a numerical simulation. The theoretical model will be used also to get the capacities of a dial system in fire surveillance of wooded areas. Fixing the burning rate for several fuels, the maximum range of detection will be evaluated. The results of these simulations will be reported in the paper.

In-cell measurements of smoke backscattering coefficients using a CO2 laser system for application to lidar-dial forest fire detection

In the lidar-dial method, the amount of the water vapor present in the smoke of the vegetable fuel is detected to reduce the number of false alarms. We report the measurements of the smoke backscattering coefficients for the CO2 laser lines 10R20 and 10R18 as determined in an absorption cell for two different vegetable fuels (eucalyptus and conifer). These experimental backscattering coefficients enable us to determine the error to be associated to the water vapor measurements when the traditional first-order approximation is assumed. We find that this first- order approximation is valid for combustion rates as low as 100 g/s. C 2010

Clipping the tail of a TE–CO2 laser pulse using a gas breakdown technique for high resolution chemical plume detection

In this article we present the realization of a spark-induced plasma shutter, working in free atmosphere, to clip off the nitrogen tail of a TE CO2 laser source. The clipping mechanism has been tested and gain switched spike pulses ranging between 100 and 115 ns have been obtained at 10P20-laser line. The shutter mechanism was optimized. In this way a clipping efficiency of 100% and a jitter value around 15 ns were obtained.

Sensitivity to Heavy-Metal Ions of Unfolded Fullerene Quantum Dots

A novel type of graphene-like quantum dots, synthesized by oxidation and cage-opening of C60 buckminsterfullerene, has been studied as a fluorescent and absorptive probe for heavy-metal ions. The lattice structure of such unfolded fullerene quantum dots (UFQDs) is distinct from that of graphene since it includes both carbon hexagons and pentagons. The basic optical properties, however, are similar to those of regular graphene oxide quantum dots. On the other hand, UFQDs behave quite differently in the presence of heavy-metal ions, in that multiple sensitivity to Cu2+, Pb2+ and As(III) was observed through comparable quenching of the fluorescent emission and different variations of the transmittance spectrum. By dynamic light scattering measurements and transmission electron microscope (TEM) images we confirmed, for the first time in metal sensing, that this response is due to multiple complexation and subsequent aggregation of UFQDs. Nonetheless, the explanation of the distinct behaviour of transmittance in the presence of As(III) and the formation of precipitate with Pb2+ require further studies. These differences, however, also make it possible to discriminate between the three metal ions in view of the implementation of a selective multiple sensor.

Automatic localization of backscattering events due to particulate in urban areas

Particulate matter (PM), emitted by vehicles in urban traffic, can greatly affect environment air quality and have direct implications on both human health and infrastructure integrity. The consequences for society are relevant and can impact also on national health. Limits and thresholds of pollutants emitted by vehicles are typically regulated by government agencies. In the last few years, the interest in PM emissions has grown substantially due to both air quality issues and global warming. Lidar-Dial techniques are widely recognized as a costeffective alternative to monitor large regions of the atmosphere. To maximize the effectiveness of the measurements and to guarantee reliable, automatic monitoring of large areas, new data analysis techniques are required. In this paper, an original tool, the Universal Multi-Event Locator (UMEL), is applied to the problem of automatically indentifying the time location of peaks in Lidar measurements for the detection of particulate matter emitted by anthropogenic sources like vehicles. The method developed is based on Support Vector Regression and presents various advantages with respect to more traditional techniques. In particular, UMEL is based on the morphological properties of the signals and therefore the method is insensitive to the details of the noise present in the detection system. The approach is also fully general, purely software and can therefore be applied to a large variety of problems without any additional cost. The potential of the proposed technique is exemplified with the help of data acquired during an experimental campaign in the field in Rome.

Electron Beam Welding of IN792 DS: Effects of Pass Speed and PWHT on Microstructure and Hardness

Electron Beam (EB) welding has been used to realize seams on 2 mm-thick plates of directionally solidified (DS) IN792 superalloy. The first part of this work evidenced the importance of pre-heating the workpiece to avoid the formation of long cracks in the seam. The comparison of different pre-heating temperatures (PHT) and pass speeds (v) allowed the identification of optimal process parameters, namely PHT = 300 °C and v = 2.5 m/min. The microstructural features of the melted zone (MZ); the heat affected zone (HAZ), and base material (BM) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), electron back-scattered diffraction (EBSD), X-ray diffraction (XRD), and micro-hardness tests. In the as-welded condition; the structure of directionally oriented grains was completely lost in MZ. The γ’ phase in MZ consisted of small (20–40 nm) round shaped particles and its total amount depended on both PHT and welding pass speed, whereas in HAZ, it was the same BM. Even if the amount of γ’ phase in MZ was lower than that of the as-received material, the nanometric size of the particles induced an increase in hardness. EDS examinations did not show relevant composition changes in the γ’ and γ phases. Post-welding heat treatments (PWHT) at 700 and 750 °C for two hours were performed on the best samples. After PWHTs, the amount of the ordered phase increased, and the effect was more pronounced at 750 °C, while the size of γ’ particles in MZ remained almost the same. The hardness profiles measured across the joints showed an upward shift, but peak-valley height was a little lower, indicating more homogeneous features in the different zones.